(i) Field of the Invention
The present invention relates to plant cells and plants which synthesize an increased amount of hyaluronan, and to methods for preparing such plants, and also to methods for preparing hyaluronan with the aid of these plant cells or plants. Here, plant cells or genetically modified plants according to the invention have hyaluronan synthase activity and additionally an increased glutamine:fructose 6-phosphate amidotransferase (GFAT) activity and an increased UDP glucose dehydrogenase (UDP-Glc-DH) activity, compared to wild-type plant cells or wild-type plants. The present invention furthermore relates to the use of plants having increased hyaluronan synthesis for preparing hyaluronan and food or feedstuff containing hyaluronan.
(ii) Description of the Related Art
Hyaluronan is a naturally occurring unbranched, linear mucopolysaccharide (glucosaminoglucan) which is constructed of alternating molecules of glucuronic acid and N-acetyl-glucosamine. The basic building block of hyaluronan consists of the disaccharide glucuronic acid-beta-1,3-N-acetyl-glucosamine. In hyaluronan, these repeating units are attached to one another via beta-1,4 linkages.
In pharmacy, use is frequently made of the term hyaluronic acid. Since hyaluronan is in most cases present as a polyanion and not as the free acid, hereinbelow, the term hyaluronan is preferably used, but each term is to be understood as embracing both molecular forms.
Hyaluronan has unusual physical chemical properties, such as, for example, properties of polyelectrolytes, viscoelastic properties, a high capacity to bind water, properties of gel formation, which, in addition to further properties of hyaluronan, are described in a review article by Lapcik et al. (1998, Chemical Reviews 98(8), 2663-2684).
Hyaluronan is a component of extracellular connective tissue and bodily fluids of vertebrates. In humans, hyaluronic acid is synthesized by the cell membrane of all body cells, especially mesenchymal cells, and ubiquitously present in the body with a particularly high concentration in the connective tissues, the extracellular matrix, the umbilical cord, the joint fluid, the cartilaginous tissue, the skin and the vitreous body of the eye (Bernhard Gebauer, 1998, Inaugural-Dissertation, Virchow-Klinikum Medizinische Fakultät Charité der Humboldt Universität zu Berlin; Fraser et al., 1997, Journal of Internal Medicine 242, 27-33).
Recently, hyaluronan was also found in animal non-vertebrate organisms (molluscs) (Volpi and Maccari, 2003, Biochimie 85, 619-625).
Furthermore, some pathogenic gram-positive bacteria (Streptococcus group A and C) and gram-negative bacteria (Pasteurella) synthesize hyaluronan as exopolysaccharides which protect these bacteria against attack by the immune system of their host, since hyaluronan is a non-immunogenic substance.
Viruses which infect single-cell green algae of the genus Chlorella, some of which are present as endosymbionts in Paramecium species, bestow upon the single-cell green algae the ability to synthesize hyaluronan after infection by the virus (Graves et al., 1999, Virology 257, 15-23). However, the ability to synthesize hyaluronan is not a feature which characterizes the algae in question. The ability of the algae to synthesize hyaluronan is mediated by an infection with a virus whose genome has a sequence coding for hyaluronan synthase (DeAngelis, 1997, Science 278, 1800-1803). Furthermore, the virus genome contains sequences coding for an UDP-glucose dehydrogenase (UDP-Glc-DH) and a glutamine:fructose 6-phosphate amidotransferase (GFAT). UDP-Glc-DH catalyzes the synthesis of UDP-glucuronic acid used as substrate by hyaluronan synthase. GFAT converts fructose 6-phosphate and glutamine into glucosamine 6-phosphate which is an important metabolite in the metabolic pathway for hyaluronan synthesis in, for example, bacteria. Both algeal genes encode active proteins which, like the hyaluronan synthase of the virus, are transcribed simultaneously in the early phase of the viral infection (DeAngelis et al., 1997, Science 278, 1800-1803, Graves et al., 1999, Virology 257, 15-23). The activity of a protein having glutamine:fructose 6-phosphate amidotransferase (GFAT) activity could be detected neither in extracts from cells not infected by a virus nor in virus-infected cells (Landstein et al., 1998, Virology 250, 388-396). Accordingly, the role of the expression of UDP-Glc-DH and GFAT in virus-infected Chlorella cells for the hyaluronan synthesis, and whether they are required for hyaluronan synthesis, is not known.
Naturally occurring plants themselves do not have any nucleic acids in their genome which code for proteins catalyzing the synthesis of hyaluronan and, although a large number of plant carbohydrates have been described and characterized, it has hitherto not been possible to detect hyaluronan or molecules related to hyaluronan in non-infected, naturally occurring plants (Graves et al., 1999, Virology 257, 15-23).
The catalysis of the hyaluronan synthesis is effected by a single membrane-integrated or membrane-associated enzyme, hyaluronan synthase. The hyaluronan synthases which have hitherto been studied can be classified into two groups: hyaluronan synthases of Class I and hyaluronan synthases of Class II (DeAngelis, 1999, CMLS, Cellular and Molecular Life Sciences 56, 670-682).
The hyaluronan synthases of vertebrates are further distinguished by the identified isoenzymes. The different isoenzymes are referred to in the order of their identification using Arabic numbers (for example, hsHAS1, hsHAS2, hsHAS3).
The mechanism of the transfer of synthesized hyaluronan molecules across the cytoplasma membrane into the medium surrounding the cell has not yet been fully elucidated. Earlier hypotheses assumed that transport across the cell membrane was effected by hyaluronan synthase itself. However, more recent results indicate that the transport of hyaluronan molecules across the cytoplasma membrane takes place by energy-dependent transport via transport proteins responsible for this action. Thus, Streptococcus strains were generated by mutation in which the synthesis of an active transport protein was inhibited. These strains synthesized less hyaluronan than corresponding wild-type bacteria strains (Ouskova et al., 2004, Glycobiology 14(10), 931-938). In human fibroblasts, it was possible to demonstrate, with the aid of agents specifically inhibiting known transport proteins, that it is possible to reduce both the amount of hyaluronan produced and the activity of hyaluronan synthases (Prehm and Schumacher, 2004, Biochemical Pharmacology 68, 1401-1410). In which amount, if at all, transport proteins capable of transporting hyaluronan are present in plants is not known.
The unusual properties of hyaluronan offer a wealth of possibilities for application in various fields, such as, for example, pharmacy, the cosmetics industry, in the production of food and feed, in technical applications (for example as lubricants), etc. The most important applications where hyaluronan is currently being used are in the medical and cosmetics field (see, for example, Lapcik et al., 1998, Chemical Reviews 98(8), 2663-2684, Goa and Benfield, 1994, Drugs 47(3), 536-566).
In the medical field, hyaluronan-containing products are currently used for the intraarticular treatment of arthrosis and in ophthalmics used for eye surgery. Hyaluronan is also used for treating joint disorders in racehorses. In addition, hyaluronic acid is a component of some rhinologics which, for example in the form of eye drops and nasalia, serve to moisten dry mucous membranes. Hyaluronan-containing solutions for injection are used as analgesics and antirheumatics. Patches comprising hyaluronan or derivatized hyaluronan are employed in wound healing. As dermatics, hyaluronan-containing gel implants are used for correcting skin deformations in plastic surgery.
For pharmacological applications, preference is given to using hyaluronan having a high molecular weight.
In cosmetic medicine, hyaluronan preparations are among the most suitable skin filler materials. By injecting hyaluronan, for a limited period of time, it is possible to smooth wrinkles or to increase the volume of lips.
In cosmetic products, in particular in skin creams and lotions, hyaluronan is frequently used as moisturizer by virtue of its high water-binding capacity.
Furthermore, hyaluronan-containing preparations are sold as so-called nutraceuticals (food supplements) which can also be used in animals (for example dogs, horses) for the prophylaxis and alleviation of arthrosis.
Hyaluronan used for commercial purposes is currently isolated from animal tissues (rooster combs) or prepared fermentatively using bacterial cultures.
U.S. Pat. No. 4,141,973 describes a process for isolating hyaluronan from rooster combs or alternatively from umbilical cords. In addition to hyaluronan, animal tissues (for example rooster combs, umbilical cords) also contain further mucopolysaccharides related to hyaluronan, such as chondroitin sulfate, dermatan sulfate, keratan sulfate, heparan sulfate and heparin. Furthermore, animal organisms contain proteins (hyaladherins) which bind specifically to hyaluronan and which are required for the most different functions in the organism, such as, for example, the degradation of hyaluronan in the liver, the function of hyaluronan as lead structure for cell migration, the regulation of endocytosis, the anchoring of hyaluronan on the cell surface or the formation of hyaluronan networks (Turley, 1991, Adv Drug Delivery Rev 7, 257 ff.; Laurent and Fraser, 1992, FASEB J. 6, 183 ff.; Stamenkovic and Aruffo, 1993, Methods Enzymol. 245, 195 ff; Knudson and Knudson, 1993, FASEB 7, 1233 ff.).
The Streptococcus strains used for the bacterial production of hyaluronan are exclusively pathogenic bacteria. During cultivation, too, these bacteria produce (pyrogenic) exotoxins and hemolysins (streptolysin, in particular alpha- and beta-hemolysin) (Kilian, M.: Streptococcus and Enterococcus. In: Medical Microbiology. Greenwood, D.; Slack, R C A; Peutherer, J. F. (Eds.). Chapter 16. Churchill Livingstone, Edinburgh, UK: pp. 174-188, 2002, ISBN 0443070776) which are released into the culture medium. This renders purification and isolation of the hyaluronan prepared with the aid of Streptococcus strains more difficult. In particular for pharmaceutical applications, the presence of exotoxins and hemolysins in the preparations is a problem.
U.S. Pat. No. 4,801,539 describes the preparation of hyaluronan by fermentation of a mutagenized bacteria strain (Streptococcus zooedemicus). The mutagenized bacteria strain used no longer synthesizes beta-hemolysin. The yield achieved was 3.6 g of hyaluronan per liter of culture.
EP 0694616 describes a method for cultivating Streptococcus zooedemicus or Streptococcus equi, where, under the culture conditions employed, no streptolysin, but increased amounts of hyaluronan are synthesized. The yield achieved was 3.5 g of hyaluronan per liter of culture.
During cultivation, Streptococcus strains release the enzyme hyaluronidase into the culture medium, as a consequence of which, in this production system, too, the molecular weight is reduced during purification. The use of hyaluronidase-negative Streptococcus strains or of methods for the production of hyaluronan where the production of hyaluronidase during cultivation is inhibited are described in U.S. Pat. No. 4,782,046. The yield achieved was up to 2.5 g of hyaluronan per liter of culture, and the maximum mean molecular weight achieved was 3.8×106 Da, at a molecular weight distribution of from 2.4×106 to 4.0×106.
US 20030175902 and WO 03 054163 describe the preparation of hyaluronan with the aid of heterologous expression of a hyaluronan synthase from Streptococcus equisimilis in Bacillus subtilis. To achieve the production of sufficient amounts of hyaluronan, in addition to heterologous expression of a hyaluronan synthase, simultaneous expression of a UDP-glucose dehydrogenase in the Bacillus cells is also required. US 20030175902 and WO 03 054163 do not state the absolute amount of hyaluronan obtained in the production with the aid of Bacillus subtilis. The maximum mean molecular weight achieved was about 4.2×106. However, this mean molecular weight was only achieved for the recombinant Bacillus strain where a gene coding for the hyaluronan synthase gene from Streptococcus equisimilis and the gene coding for the UDP-glucose dehydrogenase from Bacillus subtilis were integrated into the Bacillus subtilis genome under the control of the amyQ promoter, where at the same time the Bacillus subtilis-endogenous cxpY gene (which codes for a cytochrome P450 oxidase) was inactivated.
WO 05 012529 describes the preparation of transgenic tobacco plants which were transformed using nucleic acid molecules encoding for hyaluronan synthases from Chlorella-infecting viruses. In WO 05 012529, use was made, on the one hand, of nucleic acid sequences encoding for hyaluronan synthase of the Chlorella virus strain CVHI1 and, on the other hand, of the Chlorella virus strain CVKA1 for transforming tobacco plants. The synthesis of hyaluronan could only be demonstrated for a plant transformed with a nucleic acid sequence encoding for a hyaluronan synthase isolated from the Chlorella virus strain CVKA1. For tobacco plants transformed with a nucleic acid sequence encoding for a hyaluronan synthase isolated from the Chlorella virus strain CVHI1, it was not possible to detect hyaluronan synthesis in the corresponding transgenic plants. The amount of hyaluronan synthesized by the only hyaluronan-producing transgenic tobacco plant in WO 05 012529 is stated as being about 4.2 μg of hyaluronan per ml of measured volume which, taking into account the description for carrying out the experiment in question, corresponds approximately to an amount of at most 12 μg of hyaluronan produced per gram of fresh weight of plant material.
Hyaluronan synthase catalyzes the synthesis of hyaluronan from the starting materials UDP-N-acetyl-glucosamine and UDP-glucuronic acid. Both starting materials mentioned are present in plant cells.
In plant cells, UDP-glucuronic acid serves as metabolite for one of a plurality of possible paths for synthesizing ascorbic acid (Lorence et al., 2004, Plant Physiol 134, 1200-1205) and as a central metabolite for the synthesis of the cell wall components pectin and hemicellulose which are synthesized in the endoplasmatic reticulum of the plant cell (Reiter, 1998, Plant Physiol Biochem 36(1), 167-176). The most important and most frequently occurring monomer of pectin is D-galacturonic acid (2004, H. W. Heldt in “Plant Biochemistry”, 3rd Edition, Academic Press, ISBN 0120883910) which is synthesized using UDP-glucuronic acid. Furthermore, it is also possible, inter alia, to synthesize UDP-xylose, UDP-arabinose, UDP-galacturonic acid and UDP-apiose, metabolites for the synthesis of hemicellulose and pectin, using UDP-glucuronic acid (Seitz et al., 2000, Plant Journal, 21(6), 537-546). In plant cells, UDP-glucuronic acid can be synthesized either via the hexose phosphate metabolism comprising, inter alia, the conversion of UDP-glucose into UDP-glucuronic acid by UDP-Glc-DH or by the oxidative myo-inositol metabolism comprising the conversion of glucuronate 1-phosphate into UDP-glucuronic acid by glucuronate 1-phosphate uridilyl transferase. Both metabolic paths for synthesizing glucuronic acid appear to exist independently of one another and alternatively in different tissues/development stages of Arabidopsis plants (Seitz et al., 2000, Plant Journal 21(6), 537-546). The respective contribution of the two metabolic paths mentioned (hexose phosphate or oxidative myo-inositol metabolism) towards the synthesis of UDP-glucuronic acid has not yet been elucidated (Kärkönen, 2005, Plant Biosystems 139(1), 46-49).
The enzyme UDP-Glc-DH catalyzes the conversion of UDP-glucose into UDP-glucuronic acid. Samac et al. (2004, Applied Biochemistry and Biotechnology 113-116, Humana Press, Editor Ashok Mulehandani, 1167-1182) describe the tissue-specific overexpression of a UDP-Glc-DH from soybean in phloem cells of Alfalfa with the aim to increase the pectin content in the stems of these plants. The activity of UDP-Glc-DH, compared to the corresponding wild-type plants, was increased by more than 200%, however, the amount of pectin produced by the corresponding plants was lower than the amount of pectin produced by the corresponding wild-type plants. The amount of xylose and rhamnose monomers in the cell wall fraction of the transgenic plants in question was increased, whereas the amount of mannose monomers in the cell wall fraction was reduced.
The constitutive overexpression of a UDP-Glc-DH in Arabidosis plants resulted in aberrant growth of the plants in question compared to the corresponding wild-type plants and a dwarf phenotype. The cell wall fraction of the corresponding plants had an increased amount of mannose and galactose and a reduced amount of xylose, arabinose and uronic acids compared to the corresponding wild-type plants (Roman, 2004, “Studies on The Role of UDP-Glc-DH in Polysaccharide Biosynthesis”, PhD thesis, Acta Universitatis Upsaliensis, ISBN 91-554-6088-7, ISSN 0282-7476). Thus, these results contradict at least in part the results of Samac et al. (2004, Applied Biochemistry and Biotechnology 113-116, Humana Press, Editor Ashok Mulehandani, 1167-1182) who detected a reduced amount of mannose and an increased amount of xylose in the cell wall fraction of corresponding transgenic plants.
For the synthesis of UDP-N-acetylglucosamine in plant cells, WO 98 35047 describes a metabolic path where glucosamine is converted by a number of successive enzymatically catalyzed reaction steps with formation of the metabolites N-acetyl-glucosamine, N-acetyl-glucosamine 6-phosphate, N-acetyl-glucosamine 1-phosphate into UDP-N-acetylglucosamine. An alternative metabolic path comprises the reaction of fructose 6-phosphate and glutamine giving glucosamine 6-phosphate which is subsequently converted by a number of successive enzymatically catalyzed reaction steps with formation of the metabolites glucosamine 1-phosphate and N-acetyl-glucosamine 1-phosphate into UDP-N-acetylglucosamine. The conversion of fructose 6-phosphate and glutamine into glucosamine 6-phosphate is catalyzed by a protein having glutamine:fructose 6-phosphate amidotransferase (GFAT) activity (Mayer et al., 1968, Plant Physiol. 43, 1097-1107).
WO 00 11192 describes the endosperm-specific overexpression of a nucleic acid molecule of corn encoding for a protein having the enzymatic activity of a GFAT in transgenic corn plants with the aim to synthesize a cationic starch in plants which has 2-amino-anhydroglucose molecules. The metabolic path described which, according to the description of WO 00 11192, should result in 2-amino-anhydroglucose being incorporated into the starch, comprises inter alia the incorporation of UDP-glucosamine by starch synthases and/or glycogen synthases into the starch. It is stated that increased amounts of UDP-glucosamine could be detected in flour from endosperm of the transgenic corn plants in question overexpressing a nucleic acid molecule encoding for a protein having the (enzymatic) activity of a GFAT translationally fused with a plastid signal peptide. When the protein having the (enzymatic) activity of a GFAT was expressed without signal peptide, it was possible to detect an increased amount of glucosamine 1-phosphate in the corresponding flours from corn endosperm tissue. It was not possible to detect cationic starch in the transgenic plants.
The production of hyaluronan by fermentation of bacteria strains is associated with high costs, since the bacteria have to be fermented in sealed sterile containers under expensive controlled culture conditions (see, for example, U.S. Pat. No. 4,897,349). Furthermore, the amount of hyaluronan which can be produced by fermentation of bacteria strains is limited by the production facilities present in each case. Here, it also has to be taken into account that fermenters, as a consequence of physical laws, cannot be built for excessively large culture volumes. Particular mention may be made here of homogeneous mixing of the substances fed in from the outside (for example essential nutrient sources for bacteria, reagents for regulating the pH, oxygen) with the culture medium required for efficient production, which, in large fermenters, can be ensured only with great technical expenditure, if at all.
The purification of hyaluronan from animal organisms is complicated owing to the presence, in animal tissues, of other mucopolysaccharides and proteins which specifically bind to hyaluronan. In patients, the use of hyaluronan-containing medicinal preparations contaminated by animal proteins can result in unwanted immunological reactions of the body (U.S. Pat. No. 4,141,973), in particular if the patient is allergic to animal proteins (for example chicken egg white). Furthermore, the amounts (yields) of hyaluronan which can be obtained from animal tissues in satisfactory quality and purity are low (rooster comb: 0.079% w/w, EP 0144019, U.S. Pat. No. 4,782,046), which necessitates the processing of large amounts of animal tissues. A further problem in the isolation of hyaluronan from animal tissues consists in that the molecular weight of hyaluronan during purification is reduced since animal tissues also contain a hyaluronan-degrading enzyme (hyaluronidase).
In addition to the hyaluronidases and exotoxins mentioned, Streptococcus strains also produce endotoxins which, when present in pharmacological products, pose risks for the health of the patient. In a scientific study, it was shown that even hyaluronan-containing medicinal products on the market contain detectable amounts of bacterial endotoxins (Dick et al., 2003, Eur J. Opthalmol. 13(2), 176-184). A further disadvantage of the hyaluronan produced with the aid of Streptococcus strains is the fact that the isolated hyaluronan has a lower molecular weight than hyaluronan isolated from rooster combs (Lapcik et al. 1998, Chemical Reviews 98(8), 2663-2684). US 20030134393 describes the use of a Streptococcus strain for producing hyaluronan which synthesizes a particularly pronounced hyaluronan capsule (supercapsulated). The hyaluronan isolated after fermentation had a molecular weight of 9.1×106 Da. However, the yield was only 350 mg per liter.
Some of the disadvantages of producing hyaluronan by bacterial fermentation or by isolation from animal Ussues can be avoided by producing hyaluronan using transgenic plants; however, the currently achieved amounts of hyaluronan which can be produced using transgenic plants would require a relatively large area under cultivation to produce relatively large amounts of hyaluronan. Furthermore, the isolation or purification of hyaluronan from plants having a lower hyaluronan content is considerably more complicated and costly than the isolation or purification from plants having a higher hyaluronan content.
Although hyaluronan has unusual properties, it is, owing to its scarcity and the high price, rarely, if at all, used for industrial applications.